Canning 101: Understanding Acid and pH in Boiling Water Bath Canning

Today’s post is inspired by a rash of questions I’ve gotten recently in regard to my recipe for Honey-Sweetened Peach Vanilla Jam. A number of you are concerned because while that recipe contents lemon zest, it doesn’t contain any lemon juice. That jam is safe as written, but we need to dig a little deeper into canning science to understand why. Read on!

If you’ve been canning for any length of time, you’ve probably heard mention of acid levels in relation to safe boiling water bath canning. Anything that is preserved in a boiling water bath must have a high acid content. The reason that high acid levels are important is that the presence of acid inhibits the germination of botulism spores into the botulism toxin. Botulism spores can only develop into the botulism toxin in low acid, oxygen-free environments.

When you preserve something in a boiling water bath canner, you heat the jars and their contents to the boiling point (that temperature varies depending on your elevation, but at sea level the boiling point is 212 degrees F). That heat is enough to kill off the micro-organisms that can cause spoilage, mold, or fermentation, but it’s not enough to kill botulism spores (they require far higher temperatures). The process of boiling the jars also helps to drive the oxygen out of the jars, creating a vacuum seal. For jars that have sufficient acid content, the result is a jar of food that is safely preserved and shelf stable.

The way food scientists (and home canners) determine whether something is high or low in acid is by pH. If something has a pH of 4.6 or below, it is deemed high in acid and is safe for boiling water bath canning. If the pH is 4.7 or above, it is considered low in acid. We’ll talk more about how to preserve those foods that are low in acid and have a pH of 4.7 or above another day, but to give you just a hint, that’s often where a pressure canner comes in.

If a food is close to the 4.6 pH point, you can often add enough acid to bring that product into the necessary safe zone. Fruits like tomatoes, figs, asian pears, melons, persimmons, papaya, white peaches and white nectarines, and bananas are often just a bit too low in acid in their natural state for safe canning. So in order to lower the pH to a safe level, we add either bottled lemon or lime juice, or powdered citric acid to products featuring those ingredients. Once the acid levels are high enough to inhibit the botulism spore’s ability to germinate into a deadly toxin, that product is safe for boiling water bath canning.

However, there are a world of foods out that naturally have a pH that is well within the zone for safe preservation in a boiling water bath canner. Here’s where we come around to the peach jam I mentioned in the introduction to this post. That recipe specifically calls for yellow peaches, which typically have a pH of 3.4 to 3.6. I know the general pH range for yellow peaches because the FDA provides a handy reference page on their website that lists the general pH range of most common fruits and vegetables.

You could certainly add lemon juice to my jam in order to balance the flavor and add a little extra pectin (citrus fruit is naturally high in pectin), but it’s not necessary for safety.

Updated to add: One last thing! It’s important to remember the pH of the entire jar counts here. This is why it’s so vital to follow tested, reliable recipes for things like tomato sauce or salsa. Sure, you can add bottled lemon juice to your tomatoes to lower the pH, but if you’ve also added onions, garlic, and basil to your sauce, you’re not just balancing the acid of the tomatoes, you’re also taking the rest of the ingredients into account. That’s why salsa recipes designed for canning contain so much bottled lemon or lime juice, or vinegar.

Great information — and thanks for the FDA link. So, if you want to find out the acidity of a specific recipe do you use the chart and and lot of arithmetic (not my strong suit) or get some litmus paper (I don’t even play a chemist on TV) or ??? Realizing, of course, that if there is any doubt, don’t do it.

I do like to can salsa, though. Most recipes measure cups instead of weight, so I’m always wondering if I’m chopping onion or peppers or cilantro to the right size and how densely the cups should be packed. The vinegar and lime juice certainly help, but I do wonder.

I made some green tomato salsa last year that was pretty good, at least initially. Everything sealed properly and stayed sealed, but six months in when I opened a jar it tasted of mold. Yikes!! Of course I threw it out, and wondered if the mold had been there in some of the tomatoes early on (I picked them in late Oct. when it was clear they weren’t going to ripen on the vine. It had been one of those Oregon Summers). I know you can’t taste botulism, and the moldy taste didn’t disrupt my digestion, but kind of freaky none the less.

The big problem with mold is that as it grows it raises the pH of the canned-good allowing growth of the C. botulinum bacteria (which cause botulism). Any sign of mold in a jar and definitely thrown it out.

If you have canned something and are curious as to whether its ph level was safe when canned, can you test the ph level when you *open* the jar later on? What I mean is, can you open it, use litmus paper to test it? Would the reading from the paper be enough to determine whether the ph when canned was enough?

Bottled lemon juice has a established ph level. Fresh lemon juice varies from lemon to lemon, therefore there is no way of knowing what the ph level is. So if you are adding lemon juice to increase your ph level, you need to use the bottle lemon juice. Hope that helps.

Thanks for this Marisa. It’s great to get a refresher course now and again. It helps me to hear it in simple terms that I can explain to others who are new to canning in a way that makes sense and keeps them safe.

There are other lists. If you search for “pH of common foods” you’ll find a number of them. I have done some pH testing in the past, but typically try to find information developed by others before resorting to that.

Thanks Marisa
I’m new to canning and checked out your “Food In Jars” book from the library. I am flooded with cherry tomatoes this year and was thrilled to see you had recipes that didn’t require peeling and seeding. I plan on trying your basic tomato salsa and the roasted corn salsa. Being new to canning, I appreciate that your recipes are for smaller quantities … I was dreading having dozens of jars of something that I might not have liked.

My question is this – can one use litmus paper as a good test to decide if one could use water bath canning for something, or is this not reliable enough? I made the roasted tomatillo salsa from your recipe on this site, which was delicious, but I did not can it, per your caution. I then made another one designed for canning – it had a lot of vinegar and, while good, was not as delicious as the first one. I would have loved to have canned the first one. Seems to me a litmus test would be good enough – but maybe not?

Great information! Wish I would have read it last week. I canned some homemade chicken broth, after reading it looks like I should have used a pressure canner and strained thru a cheesecloth- neither of which I did. I took my fresh broth (fat and all) and canned it for 30 min in a boilng water bath. I am pretty new to canning as I have made and canned a few salsa’s and some relish but this is what I get for trying something new! LOL Any help would be great! Thanks

I want to make a batch of the tomato sauce from Animal, Vegetable, Miracle — I think you’ve mentioned making it here in the past — but the recipe calls for “2 TBSP GROUND LEMON PEEL” and I’d like to use orange peel instead, since I love the taste and I have a whole jar of orange peel on hand but no lemon peel. I can’t find a source on the pH of those to tell me if this is a safe substitution.

My understanding is that the cirtus peel is not being used for acid level balance, but that comes from the final note to add 2 tablespoons lemon juice to each quart. As long as you use the lemon juice, subbing orange for lemon peel is fine.

In order for me to sell my jellies at the local farmer’s market, I have to keep record of the pH of every batch of jelly. A food-grade pH meter can give you the pH of anything you want to can in just a few seconds. I got mine for < $50. Considering the amount of canning I do, it was a great investment.

Sure Sande. I use the “Checker by HANNA Instruments”. The model number on the Hanna Instruments website is 98103. It gives an accurate pH reading in a matter of seconds. One can search the internet for a source other than HANNA. I purchased mine through Amazon for $35. There is an extra cost for the calibration solutions and storage solution. One should calibrate the meter each time it is taken out of the box to use it. One word of caution; never submerge the bulb into boiling hot liquids. Measure the pH of the solution before it is heated or set aside a couple of tablespoons to cool to room temperature after ladeling the rest into jars. With proper care, a pH meter will last for a long time and is a great investment if doing a lot of canning.

I wanted to offer a little clarification about development of the Botulism spores. The spores are essentially a state of the Clostridium botulinum bacteria that exist as a sort of resting form where they may exist stably for years without growing. When the acidity is insufficient (pH is too high), the Botulism spores can activate and become reproducing C. botulinum bacteria. The growing bacteria produce the botulism toxin and cause the disease Botulism.

Hi Marisa,
wonderful site with info beyond what the extension office gives. I really appreciate it.

I have a question for you about hot sauce. Currently we make it as a fresh product (ie, we bottle it but tell friends to keep it in fridge and consume fast). Our process is to sanitize all equipment & lids, boil the sauce, clean, sanitize, and heat the bottles in oven, then funnel fill the hot bottles with the hot sauce, cap it, turn upside down to heat the lid.

I’m fine with that process for a fresh product but I’d really like to have something with a long shelf life. But it’s very traditional to use woozy bottles for hot sauce. I think the lids are actually boil-able. Should we consider a hot water boil of the bottles? The “big boys” must be doing something like that or those bottles would not be shelf stable.

Kathryn,
First the caveat – I am not a commercial manufacturer or a food scientist and I speak only from my own years of experience.
I make and bottle hot sauce at home – different varieties.
I used different red chilies that I ferment and age myself with only salt. This can naturally produce a pH of as low as 3.2 (verified by my carefully calibrated pH meter).
I combine all of my ingredients, bring the sauce to a boil and simmer for 2 minutes.
I cool a couple of tablespoons and check the pH. I shoot for <4.0 to be very safely below the 4.6 recommended for canning.
I bottle the sauce just like you do. Clean bottles, heated in the oven to 200F, and I funnel in the sauce (making sure the sauce is at least 180F).
I leave about 1/2" of head space, and I cap the bottle with a reducer and the plastic cap. I also invert the bottles and allow them to cool this way. I do not refrigerate afterwards.
I've given away hundreds of bottles that I packed this way with no issues.
I make a few different varieties and some of them are super-hot with ghost peppers and scorpion peppers, so a little goes a long way. I have had open bottles in the kitchen cabinet (not the fridge) for a year and they still taste great. The color changes a bit from the original due to oxidation, but that is about it.
I also have bottles that were unopened on the shelf for a year, and when I open them, they are as good as the day I bottled them.
Again – this is just my experience. It is hard to find good solid info on this online, but not impossible.
Good Luck!

Your website is so awesome! So, thanks for that. Being new to canning, this post got me thinking. I am a huge fan of ginger, and lots of it. I love making applesauce and pear sauce and throwing in a heap of it. But in reading your post, I’m wondering if that’s not such a good idea. All the recipes I’ve seen with ginger in them call pretty small amounts of fresh ginger, or ginger powder. If I go throwing in a couple of big chunks, will I be throwing off the PH? I did use a bunch of lemon juice as well, so I’m hoping it balances out?

Great article! I’ve been canning for a year or so and I’m beginning to sell to friends and family who can’t get enough! I want to create new recipes and need a pH tester. After checking out the FDA link, in theory if I only use high ph level foods without addition, based in treated recipes (for sugar amounts etc) would I even need t test these recipes?

Oh you may be able to help me Marisa! I’m new to canning fruit. Usually can and not waterbath. I had a great fig harvest this year. In short I didn’t follow the recipe, I got on another blog. Huge mistake, #1 but here’s my quandary… recipe called for 4 lbs of figs per 4 cups of unsweetened apple juice. I put 24 pounds! (Yes pounds) mistake # 2 in a huge pot and began simmering…but I only had 8 cups of Apple juice. I figured only about 4 more cups would fit anyway (no biggie-I think bad mistake #3) anyway balsamic vinegar, lemon juice, vanilla etc went in the right proportions but as it began bubbling I immediately noted a fermented smell.

So… is this safe? It has sat in various places, inside on the counter, outside on a table, then back in the fridge. I know… just having a really difficult time throwing that many figs out! After sitting out all night then the next day, the taste has subsided. I refrigerated a small portion that I immediately got out and blended. It was my taste test batch that tasted horribly like a cheap wine. That batch has been in my fridge and now after 3 days, tastes almost normal with very little ferment taste. What and how is this? So what should I do. Please help and thanks in advance.

I’ve thus far avoided peach jam because I don’t feel like peeling a boatload of peaches. **shrugs** I know you have a “Lazy Peach Jam” so maybe I ought to give it a try anyway with peels on :).

I bought 2.5 pounds of yellow nectarines yesterday, planning to make a small batch of stonefruit jam that does NOT require peeling! As long as I follow a trusted source’s recipe for peach or apricot jam, ensuring I add some bottled lemon (lime) juice along the way, it sounds like I’m safe. But I’m really really curious — how come I can’t seem to find any trustworthy nectarine jam recipes out there? Strangely, the National Center for Home Food Preservation only lists a preserve (whole fruit + syrup), no jam. I forgot to look it up in my Ball Blue Book — but I’ve yet to follow a recipe from there because it always calls for TOO MUCH SUGAR.

Side note: My grandma used to can the peach peelings separately and then make handpies with them. They were probably my favorite preserved fruit of hers. Something about the peachy flavor and the texture — I have never had anything since that even came close. Okay, they were a strange brownish color (oxidation I presume), but amazing.

[…] Food-borne botulism that results from home-preserved foods typically occurs in low-acid, high-moisture foods, such as tomatoes, green beans, and beets. Citrus-based marmalade is a high-acid food. The boiling process reduces the moisture content significantly. Sterilizing the jars by boiling them for at least 10 minutes is essential to reducing any possible contamination as well. For more information about food-borne botulism, check out this article on botulism from the CDC and this detailed post about canning from Food in Jars. […]

[…] your grandmother may not be safe. Botulism is a real concern with canned goods, even today, so it is important to follow temperature, acidity, and cook time guidelines in well-tested recipes. The below resources can teach you about canning in general, but be aware […]

The rules have changed! If you do high acid canning, the FDA has new rules for testing ph. I just had my inspection today. I sell canned goods and now anything with a 4.0 ph has to be batch tested at least 4 times per year with ph test papers or a ph meter. Anything above 4.0 to 4.6 needs to be per batch tested with a ph meter. Mind you, this is after initial testing at a certified lab. You must have standardized recipes and have HACCP plans for your recipes. Oh, and you need to have a log book, your canned goods must be labeled, and have batch codes (I am a home processor.)

Getting ready to test the Ph on this sauce to see if it can be canned: 1/3c honey, 3 TB minced chipotle chiles in adobo, 3 TB balsamic vinegar, 2 TB dijon mustard, 1/2 c fresh lime juice, 1 1/2 TB minced garlic, 1 tsp allspice, 1/4 c chopped cilantro, 1 tsp kosher salt, black pepper…if the levels are ok for the ph, I was going to pressure can this but was unsure of the pressure and times. Using half pint jars at 4700′ elevation. Any thoughts?

[…] Food-borne botulism that results from home-preserved foods typically occurs in low-acid, high-moisture foods, such as tomatoes, green beans, and beets. Citrus-based marmalade is a high-acid food. The boiling process reduces the moisture content significantly. Sterilizing the jars by boiling them for at least 10 minutes is essential to reducing any possible contamination as well. For more information about food-borne botulism, check out this article on botulism from the CDC and this detailed post about canning from Food in Jars. […]

[…] your grandmother may not be safe. Botulism is a real concern with canned goods, even today, so it is important to follow temperature, acidity, and cook time guidelines in well-tested recipes. The below resources can teach you about canning in general, but be aware […]